Guiding Light

Guiding Light

A new kind of glass pane containing liquid crystal
droplets can guide and manipulate beams of light.

July
25, 2003: The Information Age rides on beams of carefully
controlled light. Because lasers form the arteries of modern
communications networks, dexterous manipulation of light underpins
the two definitive technologies of our times: telecommunications
and the Internet.

Now researchers at Harvard University have developed a new way
of steering and manipulating light beams.

Using droplets of liquid crystals--the same substance in laptop
displays--the scientists can make a pane of glass that quickly
switches from transparent to diffracting and back again. When
the pane is transparent a laser beam passes straight through,
but when the pane is diffracting, it splits the beam, bending
it in several new directions.

The change is triggered by applying an electric field,
so the pane could easily be controlled by the electric signals
of a computer, offering a powerful new way to steer beams of
light.

"Telecommunications could be one application, but at
this point we're still looking at the basic properties of these
droplets. Their potential is great, and it's hard to imagine
all the ways engineers might use them," says David Weitz,
Gordon McKay Professor of Applied Physics at Harvard University
and lead scientist for the NASA-supported research.

Beyond telecommunications, one could imagine this light-steering
ability being useful in astronomy. For example, these liquid-crystal
panes could be used in reverse to combine (rather than split)
beams of light from multiple telescopes. Combining light from
many telescopes, a technique called interferometery, is a good
way to search for distant planets around other stars.

Another application:
a liquid crystal pane held in front of the mirror of a telescope
could be used to "unwrinkle" light that has passed
through Earth's turbulent atmosphere. Such adaptive optics telescopes
could gain a crystal-clear view of the heavens from Earth's surface.

Above: Light from two or more telescopes can be combined
to reduce the glare of distant stars, revealing faint planets.
Could this be an application for liquid crystal technology? [more]

The many uses of steering light are part of the reason that
NASA recently decided to award Weitz and colleagues a grant for
this research. In addition, NASA can provide a unique environment
for experimenting with liquid crystals: low gravity.

"We've already seen several exciting results from fluid
physics experiments done in Earth orbit," says Brad Carpenter,
lead scientist for NASA's Physical Sciences Research Division. "This
latest project of Dr. Weitz, who has already completed some successful
experiments on the International Space Station (ISS), was selected
for funding with the vision of aiding advances in optical information
technologies."

All droplets are created equal

Liquid crystals are a class of liquids whose molecules are
more orderly than molecules in regular fluids. Because of this
orderliness, when these liquids interact with light, they can
affect the light like crystals do.

Left: The droplets of liquid crystal in the Harvard
group's experiments, like those shown here, are of equal size
and arranged in a regular pattern. Image courtesy Harvard University.

A technique invented by Weitz and his colleagues produces
equal-sized droplets of liquid crystal, each about a dozen microns
across (a micron is one thousandth of a millimeter). Because
they're all the same size, packing the droplets together on a
glass plate causes them to arrange themselves into a honeycomb
pattern.

Making droplets of liquid crystals is nothing new; the basic
technology has been around since the mid-1980s. Today you can
find such droplets in the window-walls of some executives' offices.
With the flip of a switch, the office's transparent windows magically
change to opaque walls somewhat like frosted glass.

"The big difference between what we do and what has been
done before is that older-style glass panes contain a random
distribution of drops and drop sizes--tiny ones and big ones.
They're not ordered at all," explains Darren Link, one of
the scientists on the research team.

Without any order in the
drop size and spacing, these older liquid crystal systems simply
scatter light in all directions--hence the frosted-glass effect.

"In our case, because we make all the drops the same size,
we're able to steer light in specific directions," Link
says.

Left: Light striking a surface after passing through
the flat sheet of liquid crystal droplets doesn't appear as a
single dot, but is split and bent to produce a patterns of dots.
Image courtesy Harvard University.

The molecules in a liquid crystal droplet are long and rod-shaped.
An electric field can steer these rods (much as a magnetic field
moves a compass needle) and so control how they guide rays of
light passing through them.

Steering light at will is useful enough, but Link suspects that
the most interesting results will come from the next phase of
their research.

Exploring another dimension

"Where I think the interesting new physics is going to be
is in getting away from having a 2-D sheet and going into 3-dimensional
ordered structures," Link says. "From now on our research
will focus on doing this with real 3-D ordered structures using
smaller particles."

Link and colleagues aren't sure what they're going to find when
they shine light through several stacked-up layers of these ordered
droplets ... that's what's so exciting about doing it! It might
split the light up into a rainbow, like a prism, or it might
affect the light in a totally unexpected way.

But
first they need to find reliable ways to arrange the droplets
into various 3-dimensional patterns. This is where low gravity
comes in handy.

Weightlessness greatly simplifies making 3-D structures from
fluid droplets. The tiny droplets have a different density than
the liquid in which they are suspended. On Earth they'll either
float or sink, which greatly complicates arranging them into
a pattern. In orbit, the lack of buoyancy allows droplets to
remain nicely suspended, allowing researchers to explore many
configurations that would be difficult or impossible to create
on the ground.

Above: 3-dimensional stacked layers of liquid crystal
droplets could have some novel and useful effects on light that
passes through them. Image courtesy NASA.

Weitz says they intend to design a space-experiment and eventually
fly it on the ISS. First, though, more research on the ground
is needed to understand the basic physics of these droplets--how
they respond to the applied electric field, and exactly how those
responses affect the passing light. It's details such as these
that could soon give researchers a new tool to use in their ever-expanding
mastery of light.

Web Links

More detailed information -- a short paper offering a more technical
explanation of the concept discussed in the article above